A semiconductor device which displays an image by active matrix driving (hereinafter such a device is also referred to as an active matrix display device) has been widely used. The semiconductor device includes a pixel portion in which an image is displayed and a driver circuit which controls displaying of an image in the pixel portion. Specifically, in the semiconductor device, an image signal input to a plurality of pixels arranged in matrix in the pixel portion is controlled by the driver circuit and an image is thus displayed. Note that the driver circuit and the plurality of pixels provided in the pixel portion each have a transistor.
It is desirable from the standpoint of reducing manufacturing cost and reducing the size and the weight of such a semiconductor device that a transistor included in each pixel and a transistor included in a driver circuit or part of the driver circuit be formed over the same substrate. Note that the characteristics required for these transistors are different from each other. For example, the transistor included in each pixel is required to be formed over a glass substrate (formed by a low-temperature process) and the transistor included in the driver circuit is required to be driven at high frequencies. Therefore, a transistor which can be formed by a low-temperature process such as plasma CVD is preferable as the transistor included in each pixel, and a transistor with high field-effect mobility is preferable as the transistor included in the driver circuit.
Typically, a transistor including amorphous silicon is preferable as the transistor included in each pixel, and a transistor including single crystal silicon or polycrystalline silicon is preferable as the transistor included in the driver circuit. However, it is difficult for the transistor including amorphous silicon to be driven at high frequencies, and it is difficult for the transistor including single crystal silicon or polycrystalline silicon to be formed by a low-temperature process.
In view of the foregoing problems, a technique called polyphase driving (hereinafter also referred to as data division driving) has been developed. Data division driving is a technique in which an image signal input from the outside is divided into a plurality of image signals, and the plurality of image signals are concurrently input to a plurality of pixels. Thus, the operating frequency of the driver circuit can be reduced. For example, when an image signal is divided into four, the operating frequency of a driver circuit can be reduced to a quarter of the operating frequency prior to the signal division.
However, in the case of performing data division driving, a driver circuit has a complex structure; for example, the number of wirings which supply image signals to a pixel portion (hereinafter, such wirings are also referred to as data signal lines) is increased. Thus, wirings which supply image signals to pixels (hereinafter, such wirings are also referred to as source signal lines) might vary in wiring resistance and parasitic capacitance. In that case, image signals input to pixels vary due to the difference among the source signal lines in wiring resistance and parasitic capacitance, whereby a stripe pattern might be formed in a displayed image.
In Patent Document 1, a technique of suppressing formation of a stripe pattern is disclosed. Specifically, a technique of suppressing formation of a stripe pattern in a displayed image by leveling wiring resistances and parasitic capacitances in a plurality of source signal lines is disclosed in Patent Document 1.